Testing bottom-emitting VCSELs

ABSTRACT

An array of bottom-emitting VCSELs, with its substrate still intact, is tested by means of a probe that includes an optoelectronic array, which is aligned and coupled to the top surface of the VCSEL array. The probe is aligned to the VCSEL array just once. The optoelectronic array includes driver circuits for energizing the VCSELs and the photodetection circuits in a predetermined sequence for detecting the back emission that leaks through the top mirror of each VCSEL. In another embodiment, this probe and method are applied to testing bottom-emitting VCSELs one at a time. The VCSELs may discrete devices or part of an array. In accordance with another aspect of our invention, an array of bottom-emitting VCSELs, with its substrate still in intact, is tested by means of a probe that includes separate electronic and photodetection arrays. The probe is aligned to the VCSEL array just once. The electronic array, which is electrically coupled to the top surface of the VCSEL array, includes driver circuits for energizing the VCSELs. The photodetection array is aligned and coupled to the bottom of the substrate in order to detect the primary bottom emission of the energized VCSELs. The photodetection array is aligned so that each detector receives the emission from a particular VCSEL, but because the substrate is relatively thick, the divergence of the bottom emission produces cross-talk; that is, the bottom emission of one VCSEL may be received by an adjacent photodetector that is supposed to detect only the emission from another VCSEL. To alleviate this cross-talk problem, the VCSELs are energized in a first predetermined sequence and/or the photodetector circuitry is turned on in a second predetermined sequence.

FIELD OF THE INVENTION

[0001] This invention relates generally to vertical cavitysurface-emitting lasers (VCSELs) and, more particularly, to methods oftesting bottom-emitting VCSELs.

BACKGROUND OF THE INVENTION

[0002] A VCSEL is a semiconductor laser in which a first multiplicity ofsemiconductor layers (e.g., Group III-V compound layers) forms an activeregion (e.g., an MQW active region), which is sandwiched between asecond and third multiplicity of layers, which form a pair of mirrors.One mirror, the bottom mirror, is formed under the active region andnearer the substrate, whereas the other mirror, the top mirror, isformed above the active region and farther from the substrate. Themirrors define a cavity resonator having its longitudinal axis orientedperpendicular to the plane of the layers. When the active region isforward biased and pumping current is applied thereto in excess of thelasing threshold, the VCSEL generates stimulated, coherent radiationthat is emitted along the resonator axis. The wavelength of theradiation is determined by the bandgap of the material used to form theactive region. Thus, for operation at relatively short wavelengths inthe range of about 800-1000 nm, the layers of the active regiontypically comprise GaAs/AlGaAs compounds epitaxially grown on anoptically absorbing GaAs substrate, whereas for operation at longerwavelengths of about 1100-1600 nm, the layers typically compriseInP/InGaAsP compounds epitaxially grown on an optically transparent InPsubstrate.

[0003] The radiation may emerge through either or both mirrors dependingon their reflectivity A VCSEL is termed a bottom-emitting device if theprimary, relatively high intensity, emission is through the bottommirror. This emission will propagate through the substrate if it isoptically transparent. In many designs the substrate is removed andhence, even if it had been optically absorbing, does not obstruct theemission through the bottom mirror. On the other hand, the secondary,much lower intensity, emission that leaks through the top mirror istermed the backside emission. Bottom-emitting VCSELs are attractivebecause they are known to facilitate flip-chip bonding. In contrast, aVCSEL is termed a top-emitting device if the primary emission is throughthe top mirror.

[0004] One important feature of VCSELs is their ability to be fabricatedin an array containing, for example, thousands of lasers. These arrayscan be used to provide a multiplicity of carrier sources in fiber opticcommunication systems; e.g., dense optical interconnect solutions forhigh-end routers, cross-connects and switching systems. Before an arraycan be employed in a communications application, or any otherapplication for that matter, it must be tested in order to determinewhether each VCSEL, as well as the overall array, satisfiespredetermined performance specifications. Defective VCSELs (i.e., thosethat do not meet specification) result in lower efficiency and wastedpower consumption. Optimally an array is tested at a time in themanufacturing process (e.g., before substrate removal or final assembly)that minimizes economic loss should the array fail to meet specificationand have to be discarded. To this end in the prior art, a top-emittingVCSEL array is probed in step-and-repeat fashion, one VCSEL at a time—avery time consuming, expensive process. The probe includes drivercircuitry for supplying the necessary bias voltage and pumping currentto the VCSEL under test and photodetection circuitry for measuring theintensity of the primary emission. Testing bottom-emitting VCSEL arraysis more problematic. For short wavelength bottom-emitting devices, thepresence of the absorbing substrate prevents making optical measurementsof the primary emission. To our knowledge, therefore, manufacturerslimit their testing of short wavelength, bottom-emitting VCSELs tomaking electrical measurements to identify shorts or open circuits ineach VCSEL, again using a step-and-repeat approach. In contrast, thesubstrate of long wavelength bottom-emitting VCSELs is transparent, butthese devices are not currently in commercial manufacture to ourknowledge.

[0005] Thus, a need remains in the art for an effective technique fortesting bottom-emitting VCSELs regardless of their wavelength ofoperation.

SUMMARY OF THE INVENTION

[0006] In accordance with one aspect of our invention, an array ofbottom-emitting VCSELs, with its substrate still intact, is tested bymeans of a probe that includes an optoelectronic array, which is alignedand coupled to the top surface of the VCSEL array. The probe is alignedto the VCSEL array just once. The optoelectronic array includes drivercircuits for energizing the VCSELs and the photodetection circuits in apredetermined sequence for detecting the back emission that leaksthrough the top mirror of each VCSEL. In another embodiment, this probeand method are applied to testing bottom-emitting VCSELs one at a time.The VCSELs may be discrete devices or part of an array.

[0007] In accordance with another aspect of our invention, an array ofbottom-emitting VCSELs, with its substrate still in intact, is tested bymeans of a probe that includes separate electronic and photodetectionarrays. The probe is aligned to the VCSEL array just once. Theelectronic array, which is electrically coupled to the top surface ofthe VCSEL array, includes driver circuits for energizing the VCSELs. Thephotodetection array is aligned and coupled to the bottom of thesubstrate in order to detect the primary bottom emission of theenergized VCSELs. The photodetection array is aligned so that eachdetector receives the emission from a particular VCSEL, but because thesubstrate is relatively thick, the divergence of the bottom emissionproduces cross-talk; that is, the bottom emission of one VCSEL may bereceived by an adjacent photodetector that is supposed to detect onlythe emission from another VCSEL. To alleviate this cross-talk problem,the VCSELs are energized in a first predetermined sequence and/or thephotodetector circuitry is turned on in a second predetermined sequence;e.g., so that all VCSELs are on concurrently and adjacent photodetectorsare not on at the same time. Alternatively, all of the photodetectioncircuitry may be turned on concurrently, and the VCSELs may be energizedin a predetermined sequence to reduce cross-talk. In another example,first groups (e.g., pairs) of VCSELs may be energized in a firstpredetermined sequence and second groups (e.g., pairs) of photodetectioncircuits may be energized in a second predetermined sequence so as toreduce cross-talk, with VCSELs in each first group being energizedconcurrently with one another and circuits in each second group beingenergized concurrently with one another.

[0008] Both aspects of our invention enable testing of an entire arrayessentially simultaneously, thereby reducing costs of testing to thepoint that it is feasible to test all VCSEL arrays prior to finalassembly. Since only VCSEL arrays that meet specification are assembled,final device yields are improved. Furthermore, and in accordance withanother embodiment of our invention, the drive circuits to the VCSELsthat do not meet specification are turned off in the final device,thereby reducing power consumption wasted on such VCSELs.

BRIEF DESCRIPTION OF THE DRAWING

[0009] Our invention, together with its various features and advantages,can be readily understood from the following more detailed descriptiontaken in conjunction with the accompanying drawing, in which:

[0010]FIG. 1 is a schematic view of apparatus for testing an array ofbottom-emitting VCSELs in accordance with one aspect of our invention;and

[0011]FIG. 2 is a schematic view of alternative apparatus for testing anarray of bottom-emitting VCSELs having a transparent substrate inaccordance with another aspect of our invention

[0012] In the interest of clarity and simplicity, the figures have notbeen drawn to scale.

DETAILED DESCRIPTION OF THE INVENTION Preferred Embodiment

[0013] With reference now to FIG. 1, apparatus 10 for testing VCSELsincludes an array 12 of bottom-emitting VCSELs 15 formed on a substrate14. The array 12 is depicted at an intermediate stage of itsmanufacture; i.e., before the substrate has been removed and beforefinal assembly. Each VCSEL comprises an active region 25 sandwichedbetween a backside reflector 17 and frontside reflector 19. The tworeflectors form an optical cavity resonator that has its longitudinalaxis essentially perpendicular to the plane of the VCSEL layers. A probein the form of an optoelectronic array 16 is aligned with and coupled tothe VCSEL array 12 just once. The coupling is both electrical andoptical. Thus, the array 16 includes driver circuits 18 for applyingforward bias voltage and pumping current to the VCSELs in apredetermined sequence. The current is sufficient to cause the selectedVCSELs to emit radiation, which below threshold is spontaneous emissionand above threshold is stimulated emission. The emission has twoprincipal parts: primary emission 21 that emerges from relatively lowreflectivity, frontside reflector 19 and secondary, lower power,emission 22 that leaks through relatively higher reflectivity, backsidemirror 17. The optoelectronic array 16 also includes photodetectioncircuitry 20, which typically includes suitable photodetectors (notshown) aligned to receive the backside emissions 22 of the VCSELs. Thephotodetector circuitry is likewise turned on in a predeterminedsequence. For example, all of the VCSELs and all of the photodetectioncircuitry may be turned concurrently. Other sequences, however, may beadvantageous as discussed infra with respect to the embodiment of FIG.2.

[0014] Electrical contacts 30 on the VCSEL array 12 and electricalcontacts 40 on the optoelectronic array 16 enable the drive circuitry 18to deliver the requisite voltage and current to the VCSEL array. Inaddition, the contacts are shaped (e.g., as annular rings) to provide anunimpeded path for the weak backside emission 22 to be received by thephotodetectors in the photodetection circuitry 20.

[0015] Collection of data from the various tests requires only a singleact of aligning the probe to the VCSEL array. The data is illustrativelydelivered to a computer 50, which calculates the various performanceparameters of the VCSELs (e.g., the threshold current, slope efficiency,impedance, optical, power, which are derived from measured L-I and I-Vcurves) and compares them to predetermined specifications. The computermay map the array to identify good VCSELs and defective VCSELs. Later,if the overall VCSEL array meets specification and is finally assembled,this map may be used to provide control signals to the drive circuitryused to actually operate the VCSEL array; i.e., in order to turn on goodVCSELs and to turn off defective VCSELs, thereby reducing powerconsumption that would otherwise be wasted on improperly functioning ornon-functioning VCSELs.

[0016] In accordance with another embodiment of our invention, theforegoing testing method may be applied to VCSELs one at a time; thatis, single, discrete VCSELs or single VCSELs in an array. One-at-a-timetesting may be done in well-known step-and-repeat fashion. Thus, at anintermediate stage of its manufacture, a bottom-emitting VCSEL is testedby measuring a the backside emission that leaks through the backsidereflector, determining whether a selected quality of the VCSEL meetspredetermined specification, and then finishing the VCSEL assembly in aconfiguration that is designed to use radiation emitted from thefrontside reflector. In final assembly the substrate is typicallyremoved.

[0017] In either case, the VCSELs may be either short wavelength devicessuch as those made from layers of GaAs/AlGaAs compounds on an opticallyabsorbing GaAs substrate for operation at wavelengths of about 800-1000nm, or may be longer wavelength devices such as those made from layersof InP/InGaAsP compounds on an optically transparent InP substrate foroperation at wavelengths of about 1100-1600 nm Other Group III-Vcompounds may also be used.

[0018] The optoelectronic array may be fabricated in a Si substrate toinclude, for example, CMOS driver and photodetection circuitry.Likewise, the photodetectors may be Si p-i-n photodiodes.

Alternative Embodiment

[0019] In accordance with another aspect of our invention, FIG. 2 showsapparatus 100 for testing an array 112 of bottom-emitting VCSELs 115formed on a transparent substrate 114. As before, the array 112 isdepicted at an intermediate stage of its manufacture. In this case,however, the drive circuitry 118 and the photodetection circuitry 120are separated from one another. The electronic array 116, which includesthe drive circuitry 118, is aligned with the top surface of the VCSELarray and is electrically coupled thereto via contacts 130 and 140 onthe VCSEL and electronic arrays, respectively. The photodetection array12, which includes photodetectors 120 (and associated detectioncircuits, not shown), is aligned with the VCSEL array 112 so that aparticular photodetector (e.g., 120 a) is nominally positioned toreceive only the primary, frontside emission (e.g., 121 a) of itscorresponding VCSEL (e.g., 115 a). However, because the VCSELs arerelatively densely packed, because their output beams tend to diverge(especially over the relatively large thickness of the substrate 114),and further because the photodetectors tend to have relatively broadphotosensitive areas, each photodetector (e.g., 120 a) may receiveunwanted, stray optical radiation (e.g., 121 b and 121 c) from adjacentor other nonadjacent VCSELs. This stray radiation or cross-talk will, ofcourse, distort the data received by the photodetector in question(e.g., 120 a), thereby providing a false measurement of the performanceof the VCSEL (e.g., 115 a) associated with that photodetector.

[0020] In order to alleviate this problem, the computer 150, or othercontroller, illustratively turns on all of the VCSELs concurrently, butturns on the photodetection circuitry in a predetermined sequence toreduce cross-talk; for example, the sequence may require that whenphotodetector 120 a is being read, all adjacent photodetectors 120 b and120 c (as well as those, not shown, in the third dimension) are off.Other sequencing algorithms are also within the scope of our invention.

[0021] Alternatively, the computer 150 may turn on all of thephotodetection circuitry concurrently, but turn on the VCSELs in apredetermined sequence in order to reduce cross-talk; for example, thesequence may require that when VCSEL 115 a is on, all adjacent VCSELs115 b and 115 c (as well as those, not shown, in the third dimension)are off. As before, other sequencing algorithms are also within thescope of our invention.

[0022] In another example, computer 150 may energize first groups (e.g.,pairs) of VCSELs in a first predetermined sequence and second groups(e.g., pairs) of photodetection circuits in a second predeterminedsequence so as to reduce cross-talk, with VCSELs in each first groupbeing energized concurrently with one another and circuits in eachsecond group being energized concurrently with one another.

[0023] This aspect of our invention is particularly useful for testingVCSELs that have transparent substrates; e.g., long wavelength VCSELsthat are formed from InP/InGaAsP compounds on InP substrates. As before,in final assembly the substrate is typically removed.

[0024] The electronic array may be fabricated in a Si substrate toinclude, for example, CMOS drivers. The photodetection array maylikewise be fabricated in a Si substrate and may include Si p-i-nphotodiodes, or it may be formed as Group III-V compound devicesincluding, for example, InGaAs p-i-n photodiodes.

[0025] It is to be understood that the above-described arrangements aremerely illustrative of the many possible specific embodiments that canbe devised to represent application of the principles of the invention.Numerous and varied other arrangements can be devised in accordance withthese principles by those skilled in the art without departing from thespirit and scope of the invention.

What is claimed is
 1. A method of testing an optoelectronic deviceincluding a VCSEL, said VCSEL having a cavity resonator formed by arelatively low reflectivity frontside reflector and a relatively higherreflectivity backside reflector, comprising the steps of: at anintermediate stage of its assembly, measuring an optical signal leakingthrough said backside reflector of said VCSEL, determining from themeasured signal whether a selected quality of said VCSEL meets apredetermined specification, and then finishing said device in aconfiguration designed to use radiation emitted from said frontsidereflector of said VCSEL.
 2. The invention of claim 1 wherein saidmeasuring step includes aligning a probe with said device and thenmeasuring radiation leaking from backside reflectors of a multiplicityof VCSELs without performing another act of aligning said probe.
 3. Amethod of testing a bottom-emitting VCSEL array at an intermediate stageof its manufacture, the VCSEL array including VCSELs each having acavity resonator formed by a relatively low reflectivity frontsidereflector and a relatively higher reflectivity backside reflector,comprising the steps of: aligning a probe with one side of said VCSELarray, said probe including electronic circuits coupled to each of saidVCSELs for causing said VCSELs to emit radiation and includingphotodetection circuits coupled to each of said VCSELs for detectingradiation leaking through each of said backside reflectors, withoutperforming another act of aligning said probe, determining from saiddetected backside radiation whether a selected quality of each VCSELmeets a predetermined specification, and then for those VCSEL arraysthat meet specification, finishing their manufacture in a configurationdesigned to use radiation emitted from said frontside reflectors.
 4. Theinvention of claim 3 wherein said intermediate stage includesfabricating said VCSEL array on a substrate and said aligning anddetermining steps are performed without removing said substrate.
 5. Theinvention of claim 4 wherein said finishing step includes removing saidsubstrate before final assembly.
 6. A method of testing abottom-emitting VCSEL array at an intermediate stage of its manufacture,the VCSEL array including VCSELs each having a cavity resonator formedby a relatively low reflectivity frontside reflector and a relativelyhigher reflectivity backside reflector, comprising the steps of:aligning a probe with said VCSEL array, said probe including a firstarray of electronic circuits coupled to one side of said VCSEL array andto each of said VCSELs for causing said VCSELs to emit radiation andincluding a second array of photodetection circuits, includingphotodetectors coupled to an opposite side of said VCSEL array and toeach of said VCSELs for detecting radiation leaking through each of saidbackside reflectors, without performing another act of aligning saidprobe, determining from said detected backside radiation whether aselected quality of each VCSEL meets a predetermined specification, saiddetermining step including energizing said electronic and photodetectioncircuits in a fashion to reduce cross-talk between VCSELs and eachphotodetector, and then for those VCSEL arrays that meet specification,finishing their manufacture in a configuration designed to use radiationemitted from said frontside reflectors.
 7. The invention of claim 6wherein said VCSELs are energized in a first predetermined sequence andsaid photodetection circuitry is energized in a second predeterminedsequence so as to reduce cross-talk.
 8. The invention of claim 7 whereinall of said VCSELs are energized concurrently, but said photodetectioncircuits are energized in a sequence that reduces said cross-talk. 9.The invention of claim 8 wherein said photodetection circuits areenergized in a sequence that turns on a particular one of said circuitswhile concurrently turning off circuits adjacent thereto.
 10. Theinvention of claim 7 wherein all of said photodetection circuits areenergized concurrently, but said VCSELs are energized in a sequence thatreduces said cross-talk.
 11. The invention of claim 10 wherein saidVCSELs are energized in a sequence that turns on a particular one ofsaid VCSELs while concurrently turning off VCSELs adjacent thereto. 12.The invention of claim 7 wherein first groups of said VCSELs areenergized in said first sequence and second groups of said circuitry areenergized in said second sequence, with VCSELs in each of said firstgroups being energized concurrently with one another and circuits ineach of said second groups being energized concurrently with oneanother.
 13. The invention of claim 6 wherein said intermediate stageincludes fabricating said VCSEL array on a substrate and said aligningand determining steps are performed without removing said substrate. 14.The invention of claim 13 wherein said finishing step includes removingsaid substrate before final assembly.
 15. Apparatus for testing anoptoelectronic device at an intermediate stage of its manufacture, saiddevice including a VCSEL having a cavity resonator formed by arelatively low reflectivity frontside reflector and a relatively higherreflectivity backside reflector, said apparatus comprising: a probeincluding a photodetection circuitry for measuring an optical signalleaking through said backside reflector of said VCSEL, and means fordetermining from the measured signal whether a selected quality of saidVCSEL meets a predetermined specification.
 16. The invention of claim 15further including means for aligning said probe with said device justonce and wherein said photodetection circuitry measures radiationleaking from backside reflectors of a multiplicity of said VCSELs. 17.Apparatus for testing a bottom-emitting VCSEL array at an intermediatestage of its manufacture, the VCSEL array including VCSELs formed on asubstrate, each VCSEL having a cavity resonator formed by a relativelylow reflectivity frontside reflector and a relatively higherreflectivity backside reflector, said apparatus comprising: a probeincluding electronic circuits coupled to each of said VCSELs for causingsaid VCSELs to emit radiation and including photodetection circuitscoupled to each of said VCSELs for detecting radiation leaking througheach of said backside reflectors, means for aligning said probe justonce with one side of said VCSEL array, and means for determining fromsaid detected backside radiation whether a selected quality of eachVCSEL meets a predetermined specification.
 18. The invention of claim 17wherein said aligning means and determining means function withoutremoving said substrate.
 19. Apparatus for testing a bottom-emittingVCSEL array at an intermediate stage of its manufacture when itssubstrate is intact, the VCSEL array including VCSELs each having acavity resonator formed by a relatively low reflectivity frontsidereflector and a relatively higher reflectivity backside reflector, saidapparatus comprising: a probe including a first array of electroniccircuits coupled to one side of said VCSEL array and to each of saidVCSELs for causing selected ones of said VCSELs to emit radiation andincluding a second array of photodetection circuits, includingphotodetectors coupled to an opposite side of said VCSEL array and toeach of said VCSELs for detecting radiation leaking through each of saidbackside reflectors, means for aligning said probe just once with saidVCSEL array, and means for determining from said detected backsideradiation whether a selected quality of each VCSEL meets a predeterminedspecification, said determining means including means for energizingsaid electronic and photodetection circuits in a fashion to reducecross-talk between VCSELs and each photodetector.
 20. The invention ofclaim 19 wherein said probe energizes said VCSELs in a firstpredetermined sequence and said photodetection circuitry in a secondpredetermined sequence so as to reduce cross-talk.
 21. The invention ofclaim 20 wherein said probe energizes all of said VCSELs concurrently,but energizes said photodetection circuits in a sequence that reducessaid cross-talk.
 22. The invention of claim 21 wherein said probeenergizes said photodetection circuits in a sequence that turns on aparticular one of said circuits while essentially simultaneously turningoff circuits adjacent thereto.
 23. The invention of claim 20 whereinsaid probe energizes all of said photodetection circuits concurrentlybut energizes said VCSELs in a sequence that reduces said cross-talk.24. The invention of claim 23 wherein said probe energizes said VCSELsin a sequence that turns on a particular one of said VCSELs whileconcurrently turning off VCSELs adjacent thereto.
 25. The invention ofclaim 20 wherein said probe energizes first groups of said VCSELs insaid first sequence and second groups of said circuitry in said secondsequence, with VCSELs in each of said first groups being energizedconcurrently with one another and circuits in each of said second groupsbeing energized concurrently with one another.